I believe it will be out by summer. I noticed they really cranked up the advertising for H-L 2 in the latest game mags. There are at least 5 different ads for it in this months PC Gamer, including the cover story. Not that that guarantees anything, but that's a good sign.
'Course, I'm one of those gamers that also believes TF2 will be the MP part of H-L 2, as a sort of concession to the community, and that's why Valve has been so secretive about MP.